Can't communicate when use 500K and 5M Bit Rate

[YZhon.1]

Hello, everyone,

I test CANFD a long time. And I found that some Bit Rate doesn't work together. For example, I use CANFD Mode, set to 1M + 5M. It work well. But same parameter at Data Bit Rate，I change to 500k + 5M， or 250k + 5M， it can't work. It always show me Bus-off. And I watch FDCAN_ECR register, TEC is 0xff. It mean some wrong on CAN Bus. 

I try to check the CANH CANL in oscilloscope. I found the signal is error at the end of CAN Message.

[jiangfan]

You may try other combinations to find if there are some clues on the issue:

500k + 1M, 250k + 1M; 500k + 2M, 250k + 2M; 500k + 1M + 2M, and so on ...

[YZhon.1 · Accepted solution]

Hi, I figure out. It is TQ problem. When we use FDCAN, The TQ of norminal bit rate and the TQ of Data bit rate need close. When I set 5Mbit/s on Data Bit Rate, TQ is 16.66ns, And set 500K on Norminal Bit Rate, TQ is 200ns, it can't work, and show Bus-off. But when I change TQ from 200ns to 100ns, FDCAN work very well.

But I don't know why.